nir: Lower int64 in a single pass.

This commit also refactors min/max lowering into one function.
10 jobs for !1224 with lower_int64 in 13 minutes and 22 seconds (queued for 12 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #414799
debian

00:00:24

 
  Build+Test
passed #414801
meson-clang

00:08:58

passed #414805
meson-clover

00:07:02

passed #414804
meson-main

00:05:56

passed #414800
meson-swr-glvnd

00:06:18

passed #414806
meson-vulkan

00:01:50

passed #414808
scons-llvm

00:04:24

passed #414807
scons-nollvm

00:03:59

passed #414802
scons-swr

00:07:50

passed #414803
scons-win64

00:09:57